The present invention generally relates to an apparatus and method for manufacturing a semiconductor wafer and, more particularly, to a chemical mechanical polishing (CMP) machine and related polishing method.
As the elements incorporated into a semiconductor device are increasingly integrated, the structure of device wires such as gate lines and bit lines continues to become multiple-layered. For this reason, step coverage between unit cells on a semiconductor substrate is increased. To reduce the step coverage between the unit cells, various methods of polishing a wafer have been developed. Among these methods, a chemical-mechanical polishing (CMP) method, which planarizes a polished surface (processing surface) of the wafer during fabrication, is widely used.
In a general CMP process, a polishing head of a CMP apparatus secures a wafer using a vacuum or surface tension and loads the wafer on an abrasive pad of a turntable. The polishing head imposes a controllable load on the wafer to hold it in tight contact with the abrasive pad. Thereafter, the polishing head may be rotated to rotate the wafer with respect to the abrasive pad of the turntable.
In order to increase the efficiency of the CMP process, the wafer should be polished at a high speed while maintaining uniform flatness. However, characteristics such as uniformity, flatness and polishing speed of the wafer are highly dependent on relative speed between the wafer and the abrasive pad, as well as the force or load of the polishing head urging the wafer against the abrasive pad. Particularly, the larger the force imposed on the wafer by the polishing head against the abrasive pad, the faster the polishing speed. Accordingly, in the case where an uneven load is imposed on the wafer by means of the polishing head, a portion of the wafer on which relatively large force is imposed will be polished at a faster rate than other portions of the wafer on which relatively small force is imposed.
Generally, the polishing head includes a flexible membrane which is adapted to pick up and release the wafer by vacuum. However, the vacuum between the membrane and the wafer often times leaks, such that during transfer, the wafer may be dropped or otherwise harmed.
To address these limitations, a polishing head with a modified structure has been proposed, which chucks/releases a wafer via vacuum holes formed at bosses that protrude from a chucking supporter of the head. However, such a polishing head introduces limitations that are shown in FIG. 1, which is a graph illustrating the resulting uneven surface of a wafer. In FIG. 1, reference character A indicates a wafer portion corresponding to the protruded bosses and reference character B indicates a wafer portion corresponding to a step projected from an edge of the supporter. Portions A and B are relatively over-polished as compared to other portions of the wafer, thereby compromising the uniformity of polishing surface of the wafer.
Polishing uniformity in the CMP process depends highly upon the equipment used, particularly the structure of the polishing head. For this reason, the CMP industry has eagerly developed and applied membrane-type heads of a high polishing uniformity. Further, as the wafer caliber becomes larger, there is a high demand for equipment adapted for controlling the CMP polishing characteristics at regions near the edges of the wafer.
It is an object of the present invention to provide an improved polishing apparatus and method for polishing a semiconductor wafer with high polishing uniformity.
It is another object of the present invention to provide a polishing apparatus and method capable of variably controlling the pressure applied to regions of the wafer during the polishing process.
It is still another object of the present invention to provide a polishing apparatus and method capable of variably controlling the polishing speed at regions of the wafer during a polishing process.
It is yet another object of the present invention to provide a polishing apparatus having a head capable of stably securing a wafer.
In one aspect, the present invention is directed to an apparatus for polishing a wafer. The apparatus includes a base having a polishing pad; and a polishing head comprising a carrier and a membrane, the polishing head positioned over the polishing pad of the base. The polishing head includes: a supporter at an internal portion of the carrier forming a sealed region together with the membrane. A chucking ring vacuum-chucks a wafer, the chucking ring being positioned between the carrier and the supporter. Means are provided for moving the chucking ring in a vertical direction relative to the supporter.
The means for moving is preferably positioned between the carrier and the chucking ring, and includes an elastic member which is expanded by an externally provided pressure to move the chucking ring in the vertical direction. An external surface of the chucking ring is preferably covered by the membrane.
The membrane may be divided into first and second regions each enclosing sealed volumes together with the carrier, and an internal pressure of each respective first and second region is independently controlled relative to the other. The first region is preferably positioned at a center of the membrane, and the second region is positioned about the first region. The first region has a first width that is smaller than a second width of the second region.
The membrane preferably has a vacuum hole for chucking/releasing a wafer and a partition wall for dividing the membrane into first and second regions. The vacuum hole can be formed at the first region of the membrane, or the second region of the membrane.
In another aspect, the present invention is directed to an apparatus for polishing a wafer. The apparatus includes a base having a polishing pad. A polishing head comprises a carrier and a membrane communicating with the carrier so as to form first and second regions. The polishing head positioned over the polishing pad of the base. The polishing head includes a supporter at an internal central region of the carrier to provide a first chamber corresponding to the first region, and a chucking ring about the supporter in the carrier and collinear with the supporter to provide a second chamber corresponding to the second region. The membrane covers the supporter and the chucking ring.
The first chamber communicates with a first fluid passage and wherein the second chamber communicates with a second fluid passage. The supporter includes first outlets for connecting the first chamber to the first region, and the chucking ring has second outlets for connecting the second chamber to the second region.
The membrane includes vacuum holes for chucking/releasing a wafer, the vacuum holes corresponding to the second outlets of the chucking ring. The first region comprises an annular region about the center of the membrane, and the second region is positioned about the first region. A central region may be positioned within the annular first region, and the internal pressure of the central region is preferably independent of internal pressure of the first and second regions. The membrane divided into the first and second regions is preferably annular.
In another aspect, the present invention is directed to a method for polishing a wafer. A wafer is drawn by vacuum through a vacuum hole of a membrane positioned under a polishing head. The vacuum-absorbed wafer is located on a polishing pad. A fluid is injected through first and second fluid ports of a carrier on the polishing head to expand first and second independent regions of a membrane positioned under the polishing head. First and second independent pressures are thereby applied to the wafer. The polishing pad is then rotated to polish the wafer.
The fluid is preferably independently injected into the first and second fluid ports to independently apply the first and second pressures to first and second regions of the membrane. The carrier is preferably concave, and the support is at a concave interior of the carrier, and the carrier preferably includes first and second chambers and first and second chamber ports in order to uniformly and independently pass injected fluid to the first and second regions, whereby a uniform pressure is applied to the membrane during polishing.
In another aspect, the present invention is directed to a method for polishing a wafer. A vacuum is formed at a chucking ring positioned under a polishing head communicating with a first fluid port in the polishing head to position the wafer on a polishing pad. A fluid is injected into first and second fluid ports to expand first and second regions of a membrane positioned under the polishing head for applying first and second independent pressures to the wafer. The polishing pad is then rotated to polish the wafer.
The membrane may be positioned at a central portion of the polishing head, and the chucking ring may be located at an exterior of the membrane. The chucking ring can be moved in a downward vertical direction to apply a load to an edge of the wafer during the step of applying the first and second pressures to the wafer. The chucking ring is moved in the vertical direction by a pressure applied to an elastic member positioned between the carrier and the chucking ring. The chucking ring may be covered with the membrane.
In another aspect, the present invention is directed to an apparatus for polishing a wafer. A supporting portion has an abrasive pad disposed thereon. A polishing head is disposed over said abrasive pad. The polishing head comprises a carrier and at least two membranes dividing the carrier to form at least two independent chambers. A retaining ring is disposed on an edge of the polishing head. A chucking ring is disposed on a lower portion of the polishing head, wherein one of said at least two membranes encloses an outer portion of the chucking ring.